In the art of optical instruments, it is known to scan a surface to be imaged with a small light source, collect the light reflected from the illuminated spot and direct it to a detector which provides an output signal varying in time in correlation with the scanning of the illuminated spot across the surface. The detector output can be stored in a permanent storage medium or provided directly to a scanning display device, such as a television raster or a cathode ray tube display. By synchronizing the scanning operation of the illuminating source with the scanning of the display signals, a two dimensional image is produced.
One such instrument is a scanning ophthalmoscope which produces an image of the fundus of the eye. It has been found that the use of a laser light source provides improved imaging in an ophthalmoscope. A laser scanning ophthalmoscope is described in U.S. Pat. No. 4,213,678. One problem associated with ophthalmoscopes of the type described in U.S. Pat. No. 4,213,678 is that the light collected, at the time the laser is illuminating a particular area on the retina, includes not only light reflected directly from that area, but also light scattered from other surfaces and materials within the eye. This scattered light can cloud or fog the image, since it represents light contributions from other than the specific illuminated area. In an ideal system, each small illuminated area of the target object being examined produces a corresponding image area in the output display, with a brightness or intensity related only to light reflected directly from that target area. In some situations, on the other hand, the scattered light by itself, to the degree that it can be separated from the light directly reflected from the iluminated target area, is useful for diagnostic purposes.
In a device as described in the noted patent, the entrance pupil for the scanning laser beam has a small cross sectional area within the pupil of the eye, typically 0.5 mm in diameter, whereas the exit aperture for the reflected light is the overall pupil of the eye, which typically is nine mm in diameter. The detector is placed in a plane conjugate to this exit aperture. In the embodiment described in the patent, the scanning is effected by deflection galvanometers. The horizontal galvanometer is driven at 15.75 kHz. in order to match the horizontal scan frequency of a conventional television sweep, which preferably is used to display the output image. The vertical galvanometer is driven at 60 Hz to produce 525 lines per frame of the output image, again corresponding to the generation of a conventional television raster.
In a scanning ophthalmoscope of this type, the resolution in the raster display of the retinal image directly corresponds to the cross sectional area of the laser spot as it scans the retina. The contrast of the ultimate image depends, at least in part, upon the proportion of light received by the detector which is directly reflected from the illuminated area. Thus, to the extent that scattered light indirectly reaches the detector at the same time as it receives the light directly reflected from the illuminated area, the image is fogged and the contrast is reduced. The term "reflected" is used herein in a broad sense to refer to all optical energy returned by the target structure, it hence includes returned optical energy that results from both specular and diffuse reflection.
One technique used in some optical instruments to improve contrast for images of this type may be termed double scanning. According to this technique, the optical system is arranged to provide that the light reflected from the illuminated target area is selected with a scanning-like action related to the scanning of the incident illumination in such a manner that, at a given instant, the reflected light received by the detector is only that which is reflected from the illuminated target area. In effect, as applied to an ophthalmoscope, the fundus conjugate plane thereby allowing discrimination, at the conjugate retinal plane, between the light directly reflected from the retinal locus and that scattered either anteriorly or positiorly, i.e. within the retina. This approach, however, has been deemed to be unsuitable for an instrument like the laser ophthalmoscope of the type described, because in that instrument the exit aperture for the reflected light is so large that the returning reflected beam was deemed to require an unduly large scanning element. Since, at the driving frequencies associated with a television raster, a deflection galvanometer is limited by mass considerations to a very small surface, in the order of three millimeters, a reflection galvanometer large enough to encompass the returning image has been deemed not feasible.
Another deflection element which has been used for scanning optical instruments is a multifaceted rotating polygon, which would have to rotate at sufficiently high speeds to produce a horizontal scan matching the television frequencies. However, once again the size of the facet required to encompass the image received from the eye's exit aperture is prohibitively large in terms of fabricating a polygonal reflector to rotate at the required speeds.
The acousto-optical deflector is also not available in a form considered suitable for the reflected beam in such an instrument, due to aperture limitations.